One type of a conventional capacitance device comprises a SiN dielectric film sandwiched by an Al film and an N.sup.+ layer diffused with phosphorus (simply described "P" hereinafter). The N.sup.+ diffused layer is formed in a Si substrate, and is connected through an aperture of a SiO.sub.2 film provided on the Si substrate to an Al wiring. In this capacitance device, the Al film and the N.sup.+ diffused layer function as positive and negative electrodes, and the SiN dielectric film functions as a capacitance layer.
Another type of a conventional capacitance device comprises SiN and SiO.sub.2 dielectric films sandwiched by two layers of polycrystal silicon, a first layer of which is provided on a SiO.sub.2 film grown on a Si substrate, and a second layer of which is provided on the SiN dielectric film. The first layer of polycrystal silicon is connected through an aperture of a first PSG (phospho silicate glass) film provided partly on the first polycrystal silicon layer and a second PSG film to an Al wiring, and the second layer of polycrystal silicon is connected through apertures of the second PSG film to another Al wiring. In this latter type of the capacitance device, the first and second layers of polycrystal silicon which are connected to the Al wiring and film function as positive and negative electrodes, and the SiN and SiO.sub.2 dielectric films functions as a capacitance layer.
As described above, the former type of the conventional capacitance device is fabricated in a process for fabricating a semiconductor device. Therefore, the materials of the two electrodes for sandwiching the dielectric layer are different from each other to make matching with the fabricating process.
However, the former type of the conventional capacitance device has a disadvantage in that a capacitance value is changed in a state that positiveness and negativeness of a bias voltage applied across the electrodes is reverse. A leak current is also changed for the same reason. Although the cause is not clarified, it is considered that those disadvantages are resulted from the difference of material properties (for instance, work functions) of the electrode materials, the different interface states of the dielectric film (for instance, roughness) on the electrode materials, the difference of carrier concentrations caused in the electrode of the semiconductor material which is provided in the semiconductor substrate by the reverse application of the bias voltage, etc.
The latter type of the conventional capacitance device has also the same disadvantages as in the former type of the conventional capacitance device, although it has the electrodes of the same material. In this point, it is considered that those disadvantages are resulted from the aforementioned difference of the interface states, the difference of electron injecting efficiencies caused by the difference of bandgap energies of the two dielectric capacitance layers, etc.